How to add_custom_command() for the CMake build process itself?

Question

Is there any way to do the equivalent of add_custom_command (run an external script when a certain file changes), but for something that should be run during CMake script ex

Answer 1

Turning my comment into an answer

configure_file() is the right choice to retrigger the configuration process. And execute_process() is used to run commands during the configuration step.

Here is an abstract CMake snippet of the steps in your description:

function(my_add_library_from_cfg _target _cfg_file)
    # Retrigger configuration process each time config file changes
    get_filename_component(_cfg_name "${_cfg_file}" NAME)
    configure_file("${_cfg_file}" "${_cfg_name}.tmp")

    # Generating sources and file list
    execute_process(
        COMMAND ${CMAKE_COMMAND} -E echo "#define FOO" 
        OUTPUT_FILE foo.c
    )
    execute_process(
        COMMAND ${CMAKE_COMMAND} -E echo "foo.c"
        OUTPUT_FILE files.lst
    )

    # Reading file list and add library
    file(STRINGS "${CMAKE_CURRENT_BINARY_DIR}/files.lst" _sources_lst)
    add_library(${_target} ${_sources_lst} "${_cfg_file}")
endfunction(my_add_library_from_cfg)

• configure_file() will retrigger CMake each time the given .cfg file changes
  • if you actually use the .cfg file as a command line parameter in execute_process() you should use the generated .tmp version in the current binary directory
• execute_process() can do a lot of things
  • I used it to echo to stdout and write the output to files
  • the default WORKING_DIRECTORY here is the current binary directory
• file() reads in the list of source files
  • full paths each, newline separated
• I have written the generated files by purpose to the binary output directory
  • it makes a clean build easier, just delete the whole binary output directory
  • and CMake has build-in support for this; it will search for source files without absolute path first in the current source and then in current binary directory
• And I didn't use the GENERATED source file property
  • because that would only be needed if the sources were generated by a previous build step
  • and CMake itself won't recognize anymore when actually a file from source file list would be missing